1. Technical Field of the Invention
The present invention relates to a method for detecting a strain change using a fiber Bragg grating (hereinafter referred to as “FBG”) sensor and, more specifically, relates to a method for detecting an acoustic emission (hereinafter referred to as “AE”) that is caused by infinitesimal damage in a material or a structural body.
The present invention is used when the soundness of a structural body is assessed by the AE method by generating an acoustic wave with a piezoelectric element, or when a fast strain change due to a shock load is detected. The present invention can be used in the structural examination of the soundness of mechanical constructions such as automobiles, aircraft, bridges, buildings, etc.
2. Description of the Related Art
Piezoelectric elements have been conventionally used to detect AE, and a strain gauge has been used to detect a shock load.
A technique has been proposed, in which a light ray reflected from a fiber Bragg grating (FBG) sensor is transmitted through an FBG having a Bragg wavelength substantially equal to the Bragg wavelength of the FBG sensor, with the AE then detected from the transmitted light (see, I. Perez, H. L. Cui and E. Udd, 2001 SPIE, Vol. 4328, p. 209–215). Optical spectrum analyzers have been conventionally used to measure a change in the Bragg wavelength of the FBG sensor.
Since the known piezoelectric technique used to detect AE directly converts measurement parameters into an electrical signal, that technique is subject to the effect of electromagnetic interference. On the other hand, in the proposed optical technique, the FBG sensor converts measurement parameters into an optical signal, and is thus free from the effects of electromagnetic interference, however, the detected waveform does not necessarily reflect the original shape of the AE, and may sometimes suffer from distortion. Further, the known technique of measuring a change in the Bragg wavelength of the FBG sensor using the optical spectrum analyzer typically works with a sampling rate of one sample per second. For this reason, the technique is unable to track and detect a dynamic strain change.